As communication apparatuses which are connected to phone lines for the purpose of use, there are facsimile machines having a facsimile function, and so-called multi-function devices having not only the facsimile function but also other functions such as a printer function. Each of these communication apparatuses has a connection terminal, in addition to a line connection terminal which is a terminal (i.e., plug-in phone jack) for connecting the communication apparatus to a phone line. The phone connection terminal is for connection with a separate telephone outside the apparatus and is connected in parallel to the line connection terminal inside the communication apparatus. In the following description, unless otherwise noted, the term “communication apparatus” means a communication apparatus having a phone connection terminal for connection with an external telephone.
This communication apparatus determines whether an external telephone is in use or not. When it is determined that the external telephone is in use, the communication apparatus performs display on a display screen (for example, an LCD) to notify that the external telephone is in use or prevents communication through the phone line from being performed. According thereto, the communication apparatus provides convenience to both of a user of the external telephone and a user of the communication apparatus.
The communication apparatus determines whether the external telephone is in use, on the basis of whether the external telephone is in a hook-up (or off-hook) state. When the external telephone is not in the hook-up state, the communication apparatus determines that the external telephone is not in use; whereas when the external telephone is in the hook-up state, the communication apparatus determines that the external telephone is in use.
In the related art, detection on hook-up of the external telephone is performed by detecting a current flowing in the phone line. As is well known, while a telephone connected to a phone line is in an on-hook state, the phone line is open, and thus any current does not flow. Meanwhile, when the telephone becomes the hook-up state, the phone line is closed by the telephone such that a DC loop is formed between the telephone and a phone line side (a telephone exchange system), and a current flows in the phone line. Therefore, it is possible to perform detection on hook-up of the external telephone by detecting a current flowing in the phone line.
However, in the configuration according to the related art in which detection on hook-up is performed on the basis of current detection, it is necessary to provide a current detecting circuit on the primary side, and to use, for example, an optical coupler to insulating the primary side from the secondary side. For this reason, as the current detecting circuit, a large and expensive circuit should be used.
Recently, silicon data access arrangement (SDAA) has been used as an interface (line I/F) to be connected to a phone line. The SDAA has a function of detecting a voltage of a phone line (hereinafter, referred to as a line voltage), in addition to basic functions such as detection on polarity reversion, detection on various signals such as a call signal, and closing or opening of a phone line. The line voltage means an inter-cable voltage between two conductive cables constituting the phone line. Therefore, the line-voltage detecting function of the SDAA can be used to detect hook-up of the external telephone.
In other words, while the external telephone is in the on-hook state, that is, the phone line is open, the line voltage is maintained at a predetermined voltage (which is about 48V, for example, and will also be referred to as a steady voltage). Whereas, when the external telephone is hooked up such that the phone line is closed, as described above, a DC loop is formed and a current flow. As a result, the line voltage is reduced up to about half of the steady voltage (for example, several tens of voltages). Therefore, it is possible to perform detection on hook-up of the external telephone by detecting a reduction in the line voltage to about half of the steady voltage.
When the line-voltage detecting function of the SDAA is used to perform detection on hook-up of the external telephone, at least, it become unnecessary to provide a current detecting circuit on the primary side, and thus it is possible to reduce the size and cost of the communication apparatus.